Nasicon compounds are a family of compounds with high sodium conductivity. These compounds can be made into ceramics which are useful as solid electrolytes in batteries. The most commonly studied nasicon compounds have the general formula Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12. These nasicon compounds are rhombohedral at room temperature except where x is between about 1.6 and about 2.2 where they are monoclinic. While the monoclinic range of 1.6 to 2.2 is the highest conductivity range, the rhombohedral structure is more conductive than the monoclinic structure. Unfortunately, the rhombohedral structure does not form in the 1.6 to 2.2 range at room temperature, but the nasicon compounds do undergo a phase transition at a temperature between 150.degree. and 300.degree. C. in which the monoclinic structure changes to the rhombohedral structure. Thus, to obtain the highest conductivity, it is necessary to prepare the nasicon compounds in the 1.6 to 2.2 range and then use the nasicon compounds in batteries above the transition temperature. In addition to the usual difficulties which are created by having to operate at such a high temperature, the phase transition in the nasicon compound may result in volume changes which crack the ceramic and render it useless in a battery.
Another difficulty which arises with these nasicon compounds is that it is difficult to prevent the formation of two phases in the ceramic, the two phases being the nasicon structure itself and monoclinic zirconia (ZrO.sub.2). The zirconia, which is known as "free zirconia," is less conductive with respect to sodium than the nasicon phase and reduces the overall conductivity of the ceramic to the extent that it is present. Typically, about one to about 20 volume percent of a nasicon ceramic may consist of free zirconia.